The present invention relates to monitoring railway trains, and in particular to a method and apparatus used to detect the presence, speed and direction of railway trains, and to count their wheels, and measure their wheel radius and wheel flange depth.
The prior art has developed a variety of transmitter and receiver coil configuration for sensing the presence of a train wheel. Some of these are subject to errors and inaccuracies due to debris near the coils, temperature drift and component aging. Also interconnecting cables and even drift and variations in the signal processing electronics makes it difficult to guarantee the accurate detect all wheels.
Other prior art systems (Gilcher, Patent 5,333,820) use a single transmitter and a dual receiver coils in a differential bridge circuit that compensates somewhat for drift and some disturbances from debris and thermal drift. This system requires coils be mounted on both sides of the rail and require a precise balance in signal strength and a critical field adjustment to run the system at a slight imbalance in order to derive a small carrier signal. There is no adjustments for long term drift and no automatic adjustments. The use of a potentiometer to adjust the unit requires a field operation and is also subject to mechanical vibration, humidity and corrosion of the potentiometer which can lead to an undetectable error. There is also no method to automatically test that the sensor is actually operational. Hence, failure to detect a wheel is not known until a wheel passes the sensor and fails to cause the desired actions. (This circuit is hence, not vital.)
In the past, wheel sensing of variety detection means including photo-electrics, mechanical switches, load sensing, proximity switch technologies and magnetic disturbance measuring devices. All of these existing devices lack one or more of the requirements for vital railway applications, ie., critical life-preserving and accident prevention situations. These requirements are:
Reliable operation over extended temperature ranges.
Relative immunity to environmental conditions such as ice, snow, fog, chemicals, corrosion and water.
Ability to withstand intense vibration and mechanical shock generated by passing trains.
xe2x80x9cZero speedxe2x80x9d detection, ie: ability to detect a wheel even if it is moving dead-slow or stopped.
Ability to determine direction of travel in a xe2x80x9cfail-safexe2x80x9d manner.
Ability to determine sensor removal from the rail regardless of the speed of removal of the sensor from the rail.
Able to detect broken or shorted cables and defective drive electronics.
Traditional track circuits used in the railway industry employ electric currents in sections of rail separated by insulators in order to provide separate and distinct physical blocks. A differential voltage exists between the two rails when the block is not occupied by a train. When a train wheel and axles enter the block, a xe2x80x98shuntxe2x80x99 is provided which creates a change in the current and voltage which is detected by the controller. This shunting requires good electrical conductivity between the rail and the wheel, which is not always available and contributes to an unsafe condition where a train occupancy can be missed.
Traditional track circuits require that the ballast material, (e.g. the rock, gravel or slag comprising the roadbed), be non-conductive and that no other conductive material be placed between the rails. Contaminated ballast occurs frequently enough to be a serious safety hazard leading to false activation of the track circuits and/or missed train detection. The insulated joints needed to define track circuits are also troublesome, being very expensive to maintain. Furthermore, these track circuit usually indicate only occupancy and only the most complex control systems can measure the position of the train within a block.
Many alternatives to traditional track circuits have also been utilized. Photoelectric systems such as those disclosed in U.S. Pat. No. 3,581,083 to Joy will fail if snow blocks the light source. This makes them suitable only in warmer climates. Passive inductor systems such as those disclosed in U.S. Pat. No. 3,108,771 to Peling do not provide adequate signal output at low operating speeds and fail to detect slow trains. These systems cannot detect their own removal from the rail and hence are not xe2x80x9cfail-safexe2x80x9d.
A device and method of detecting the presence, the speed, direction and movement of a railway vehicle via eddy current losses induced in a train wheel as it comes into proximity of a high frequency magnetic field. The same coil is used to transmitting and receiving, increasing the reliability of the sensor. Only one side of the rail is equipped with the sensor which makes mounting easier. The single coil is automatically calibrated through a digital signal processor which adjusts automatically for all temperature, component aging and metallic debris situations as well as detecting sabotage and damaged sensors. The circuit is self calibrating and self testing such that the system can detect its own inability to detect a wheel before a wheel arrives and will enter a fail-safe state.
So testing is done by a innovative technique termed the slope test. The circuit operates at a point approximately midway on the frequency-voltage curve, and thence a purposeful incremental increase in the operating frequency can cause a slight increase in the voltage drop across the sensor. The amount of this sensor voltage change depends on the slope of the curve at the operating point. The test involves measuring the increase in voltage due to the incremental frequency increase and detecting an out of tolerance value. It is known that external factors which may change the sensors ability to detect a train will also cause a decrease in the slope of the curve. The slope values are monitored and a low reading will cause a fail-safe failure modes.
The present invention eliminates track circuits and the attendant problems and provides a method of measuring train position more accurately through the use of wheel counting sensors and innovative control software to create xe2x80x98virtual blocksxe2x80x99 that need no insulated joints, ignore ballast conductivity and provide train position information in the block.
The objectives of the invention are:
to provide a fail-safe system in all respects
to provide a train detection system which eliminates the need for wheel shunting and clean ballast.
to enhance train position measurement accuracy within a block and improve safety through enhanced collision warning capability.
to eliminate the need for insulated rail joints and provide significant maintenance cost reductions.